Apparatus and methods for vocal tract analysis of speech signals

ABSTRACT

The present invention provides for speech processing apparatus arranged for the input or output of a speech data signal and including a function generating means arranged for producing a representation of a vocal-tract potential function representative of a speech source and as an example, a speaker identification process can comprise means to capture an incoming voice signal, for example from a microphone or telephone line; means to process the signal electronically to generate a time varying series of binary vocal-tract potentials and associated non-vowel binary parameters; means to refine the signal to revoke the speaker-independent speech components; and means to compare the residual signal with a database of such residual features of known individuals.

The present invention relates to a speech processing apparatus andmethod and, in particular, but not exclusively, to such an apparatus andmethod for use within a speech recognition, speech synthesis, speechcompression or a voice identification system.

Known speech processing systems such as speech recognition systems arebased on techniques including the generation of a Hidden Markov Model(HMM) and some such systems attempt to use vocal-tract parameters toimprove performance.

One example of such a known arrangement is disclosed in U.S. Pat. No.6,236,963, which, amongst its various examples, discloses a speechrecognition system employing a generated HMM, and also a functiongeneration means for establishing a vocal-tract area function. Furtherknown research relating to articulatory levels of representation in theHMM are also known but there is no clear indication as to how suchlevels should be structured or, indeed, which articulatory parametersthe modeling should be applied to.

Such known systems are disadvantageous particularly due to theiremployment of the vocal-tract area function, which is problematic due tonon-unique mapping between the vocal-tract and the transmitted speechsignal, and so the vocal-tract area function can be seen as adisadvantageously limited descriptor of the speech signal.

In general, currently known systems such as those known from U.S. Pat.No. 6,236,963 are considered to suffer disadvantageous limitations withregard to the vocabulary size and the range of speaker characteristics,such as dialect differences, that can be handled. In general, and withregard to the operational efficiency in spontaneous speech conditions,it is found that current systems can readily fail when syllables arefound to run into each other as in natural “joined up” speech.

The problem of the definition of a compact set of control parameters forspeech acoustics remains topical due to the limitations of current HMMsystems in their dealing with the general area of phonologicalvariation, for example, continuous speech phonotactics and long-rangecontext dependencies.

The known systems can however, to some extent, be arranged to providesome form of useful functionality through the adoption of a trade-offbetween the above-mentioned potential problems. For example, a systemarranged for use with a restricted vocabulary, or only isolated phrases,can be achieved and which is somewhat speaker-independent. A simplisticform of such a known system is arranged to discriminate between “yes”and “no” responses given orally via a telephone link and which areemployed in, for example, targeted telesales services.

However, and as mentioned, such known systems are far from offering, forexample, automated speech recognition that can allow for recognitionunder spontaneous speech conditions.

The present invention seeks to provide for a speech processing apparatusand method exhibiting advantages over known such apparatuses and methodsand, in particular, one which can be employed in a speech recognitionsystem.

The invention is based upon a consideration of the physics of speechproduction with a view to defining an abstract level of representationpertinent to the phonetics phonology interface.

According to one aspect of the present invention, there is provided aspeech processing apparatus including a function generating meansarranged for estimating a vocal-tract potential function.

Advantageously, the invention provides for defining parameters as a sixbit potential function for vowel sounds.

This invention is advantageous insofar as it allows for the applicationof mathematical processing of quantum-mechanics to speech processing. Byadopting the analytical methods of quantum mechanics, the inventiontakes into account the geometric and acoustical properties of knownpotential-function types, particularly the barrier and well.Specifically, the formalism is able to quantify dispersion in regions oftract expansion and contraction, accounting for phenomena occurring atrapid changes in tract cross-section in a more accurate manner thanallowed by stepped “n-tube” models. A perturbation analysis made on thebasis of small dispersions, rather than small changes in tract area,leads to a definition of just six bitwise parameters, which combine in asimple manner to generate a 25-vowel space. Together with the generationof five or six bit consonantal feature vectors, the invention cantherefore find ready use in systems such as speech recognition andspeech synthesis.

In a further aspect, and for consonant sounds, the parameter can bedefined as in the region of five or six additional bits so as to providefor single-bit characteristic consonantal features.

Thus when also considering the consonantal feature vectors, it will beappreciated that the invention can provide for a practical eleven-bitvoice recognition system with six bits being employed for vowel sounds,and a further five for consonants.

For a potentially greater accuracy however, a twelve-bit system can beprovided employing six bits for vowel sounds and a further six bits forconsonants.

As will be appreciated the invention relates to the parameterization ofvocal-tract geometry as a potential function.

Also, in view of the advantageous back-calculation of a potentialfunction that can be achieved from an emitted wave, the speechprocessing of the present invention lends itself advantageously toeither speech recognition or speech synthesis.

Advantageously, the vocal-tract potential function is described by onegeneral function and, yet further, such general function can be employedto describe each of the internationally recognized distinct phonemes bymeans of specifying a small number of parameters of that function.

Also, each of the aforementioned parameters can comprise binaryparameters and, yet further, characteristics of the function are foundto be both speaker-specific and speaker-nonspecific.

The invention can provide for a speech processing system in which aninput sound wave is recorded and digitized and an inversion calculationperformed so as to arrive at the said potential function.

Preferably, the potential function is divided between speaker-dependentand speaker-independent sections.

The speaker-dependent sections can be arranged to be compared with thecontent of a storage means so as to perform voice identification.

Also, the speaker-independent parts can be subsequently processed toprovide for speech recognition.

Yet further, the speech recognition apparatus includes comparison meansfor comparing binary parameters of an invariant part of the sound signalwith the content of a look-up table.

Still further, means can be provided for forwarding a stream of binaryparameters into a speech parser.

Advantageously, the speech parser is arranged to confirm and/or refineinterpretation by means of, for example, grammar and/or context rules.

As an alternative, apparatus can be provided for receiving the speakerindependent parts of a potential function as the compressed, speechsignal, in addition to the speaker-dependent parts of the said, general,potential function together with voicing information for thereconstruction of a sound wave.

According to another aspect of the present invention there is provided amethod of speech processing including the step of estimating avocal-tract potential function and generating a general functiontherefrom and employing parameters thereof for representing phonemes.

Again, in this aspect of the invention, the invention provides fordefining parameters as a six bit potential function for vowel sounds,and preferably with five or six additional bits for consonant sounds.

It should be appreciated therefore that the concept underlining thepresent invention achieves its advantages through the derivation of aphysical analysis of wave propagation in the vocal-tract that is basedon quantum-mechanical scattering systems.

Advantageously therefore, the invention provides for the application ofmodern physics to the known speech inversion problem in whichvocal-tract parameters are to be identified from an acoustic signalrecorded at some point outside the mouth.

Thus, the invention can provide for a speech processing control unit, inwhich a vocal-tract potential function is derived from digitized speechby an inversion algorithm based on solution for the vocal-tract wavefunction, ψ. The invention advantageously serves to identify uniquevocal-tract parameters from a recorded acoustic signal.

The general potential function obtained is then separated into aspeaker-dependent part, which contains information about the tractlength, and may also include details of the glottal vibration, and aspeaker-renormalized part, which is obtained by algorithms such asleast-squares fit onto previous defined, mainly binary, potentialfunction strings stored in a look-up table. Vocal-tract renormalizationis implicit in the process since the binary strings have the uniquefeature of being scaled by the tract length. Information retrieved asnoted above, or individual voice characteristics obtained by othermethods, may be recombined with the compressed data for re-synthesis bywave equation methods, in a speech synthesizer.

For each of the above-mentioned purposes therefore, a practicaleleven-bit voice processing system can be provided with six bitsemployed for vowel sounds and five for consonants. Greater accuracy canbe achieved by increasing the number of bits for the consonant sounds tosix.

According to a further aspect, the invention proposes the concept ofsolving the inverse problem by means of an analysis taking theautocorrelation function of the speech signal as a basis for thesolution of the problem. In this method the running short-termautocorrelation function, over only a few glottal cycle times, reveals arelatively stable and smoothed representation of the structure of thesignal as it evolves during and between phonemes. Inversion of thesignal from this representation is particularly advantageous fordefining the consonantal feature vectors, which vary on this short timescale, and thus are not particularly well represented by Fouriertransformation over the longer sample times used in the present art.

The invention is described further hereinafter, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 is a flow diagram illustrating the concept of the presentinvention particularly as applied to speech recognition, speechsynthesis, speech compression and voice identification techniques;

FIG. 2 is a block diagram of speech recognition apparatus embodying thepresent invention;

FIG. 3 is a schematic block diagram of speech synthesis apparatusembodying the present invention;

FIG. 4 is a schematic block diagram of voice identification apparatusembodying the present invention;

FIG. 5 is a schematic block diagram of speech compression apparatusembodying the present invention;

FIG. 6 illustrates an equivalence set of area functions mapped to apotential function;

FIG. 7 illustrates an area function once a well has preceded aterminating barrier;

FIG. 8 is a graphical illustration of the phase of the constructiveeffects on a first eigenfucntion;

FIG. 9 is a graphical illustration of a destructive flattening effectwith a reverse potential configuration;

FIG. 10 is a table illustrating a six-bit vocal tract model for a 25vowel system; and

FIG. 11 comprises a vowel chart corresponding to the table of FIG. 10.

Turning first to FIG. 1, there is provided a flow diagram 10illustrating an embodiment of the present invention and, in particular,four particular aspects relating to voice identification 12, speechrecognition 14, speech compression 16 and speech synthesis 18.

As will be appreciated from the flow diagram, each of the fouraforementioned different aspects of the present invention share commonfeatures which are illustrated by the common sections of the flowdiagram.

In the flow diagram, speech data in the form of a continuous sound wave20 is recorded as digitized speech at step 22 and, in accordance withthe particularly novel feature of the present invention, an inversioncalculation is then performed on the digitized speech signal at 24 so asto derive a vocal-tract potential function.

At step 26, the potential function is separated into speaker-independentand speaker-dependent parts.

The voice identification process requires access merely to thespeaker-dependent parts of the potential function and so at step 28 suchparts are compared with stored data comprising a library of knownindividual characteristics, which comparison can lead to voice, and thusindividual, identification such as at step 30.

Returning to the main path of the flow diagram, the speaker-independentpart of the sound signal as represented by the potential function can beestimated and/or further refined at step 32 for a subsequent, andprefably binary comparison step of the invariant part of the soundsignal with data stored in a look-up table at step 34. The binaryparameter stream obtained at step 34 is retrieved for the subsequentspeech compression as illustrated at steps 36 and 38.

The step 34 will also produce a phoneme stream, which, at step 40 isdelivered to a speech parser to allow for confirmation, and/orrefinement, on the basis of standard grammar and/or context rules.

The processing continues via step 42, which represents standard finalstages in speech recognition processing so as to provide for therequired speech recognition at the step 44.

Returning to the speech compression step 38, it will be appreciated thatthe compressed data, and the parameterized control data 46 are combinedat step 48 so as to provide a stream of, prefably binary, parameters inaddition to data relating to the potential function. Such combined datais reconstructed as a sound wave at step 50 so as to provide for aspeech synthesis output at step 52.

As will be appreciated from the foregoing, a particularly importantaspect of the present invention is that the speech processing is derivedfrom a physical analysis of wave propagation in the vocal tract thatshares a framework with quantum-mechanical scattering systems. Theinvention is therefore derived from the application of modern physics tospeech inversion and in which vocal-tract geometry is sought from theacoustic signal recorded at some point outside the mouth. The definitionof a maximally small number of parameters to describe the speech signalhas long been thought to involve the vocal-tract configuration but is,in fact, an unsolved problem of Automated Speech Recognition technology.

However, as the present invention now confirms, an equation,mathematically analogous to the Klein-Gordon equation of quantummechanics, can be employed for the description of one-dimensionalacoustic systems. As will be appreciated this wave mechanical formalismleads to a unique and compact parameterization of the vocal-tractgeometry in terms of a tract potential function. While the standardknown description, in terms of a tract area function, leads to a problemof non-unique mapping between the tract and the transmitted speechsignal, for ASR technology the vocal-tract area function is aproblematic descriptor of the speech signal. The tract potentialfunction employed in its place within the present invention exhibitsadvantages of simplicity accuracy and reliability that serve to renderthe processing system of the invention particularly suited to therequirements of speech recognition, synthesis, compression and voiceidentification.

As noted previously, the process employed within the present inventionis advantageously arranged to allow for the scaling of the potentialfunction to vocal-tract length so as to achieve vocal-tractrenormalisation. Also, the method of inversion of the speech signal soas to provide the vocal-tract length and the six binary parameters forvowels, and the five or six binary parameters for consonants, mayadvantageously include the option of noise reduction algorithms such asweiner filtering and/or other steps in the processing procedure such asblind equalization, blind deconvolution or preemphasis.

Turning now to consonant sounds within the acoustic signal, the presentinvention provides for the use of a relatively small number of generallybinary parameters, such as in the order of five or six, to allow for thedescription of such consonants.

Advantageously, such five or six parameters comprise a parameter on thepotential function that represents a class of nasals, which couldcomprise vowels or consonants, and with an acoustic cue relating to thelow energy around the first harmonic frequency, and possible rapid risein frequency following a vowel. Further, the parameters can comprise aparameter serving to indicate the class of glides, a parameter servingto indicate the class of plosives and with acoustic cue relating to anabrupt drop in energy. Still further parameters can comprise a parameteron the potential function that serves to indicate the class of lateralsand a parameter that serves to represent the class of voicelessconsonants and allied with acoustic cue of aperiodic energy and a highzero-crossing rate.

With such parameters, the voicing, at the speaker glottis, of speech maybe taken as a default position.

From the above-mentioned discussion of the processing of vowel andconsonant sounds, it will be appreciated that the present invention canallow for the admission of an inventory of in the order of eleven ortwelve generally binary parameters to account for completespeaker-independent speech recognition. The use of such a number ofbinary parameters enhances the efficiency of the processing, whichefficiency can be improved even further by a reduction in the number ofparameters as follows.

As an alternative, and as discussed further below, in relation to FIG.10 of this application, the additional binary parameters for consonantsounds can be derived from the same six-bit table providingrepresentation of a 25-bit system prepared primarily as a representationof vowel sounds. In this manner, in the order of nine, rather than inthe order of ten-twelve binary parameters will then be required so as toprovide full phonetic representation which will of course lead to a yetfurther reduction in the number of parameters required for full speechprocessing and thereby lead to further increase in overall efficiency.

Although particular details of the processing required by embodiments ofthe present invention are outlined later, there now follows adescription of four different aspects of the present inventioncomprising a speech recognition system, a speech synthesis system, avoice identification system and a speech compression system illustratedin accordance with the schematic diagrams of FIGS. 2-5.

Turning first to FIG. 2, there is illustrated in block schematic form aspeech recognition system 54 including a speech capture and conversionunit 56 by which an incoming analogue speech signal is converted to adigital speech signal for subsequent processing within the speechrecognition system 54. The digitized speech signal is delivered to aninversion calculation module 58, which, in accordance with the presentinvention, is arranged to perform an inversion calculation on theincoming signal so as to derive an associated potential function.

The resulting signal from the inversion calculation module is deliveredto an optimizer module 60 which can lead to the generation of aspeaker-independent binary token stream 62 which are subsequentlydelivered to a binary string parser arrangement 64 including a parserdatabase. As required, the parser is arranged to confirm and/or refineinterpretation of the received speech signal by means of, for example,grammar and/or context rules. The output signal from the arrangement 64can then be processed in the same manner as conventional systems so asto produce, for example, a recorded, or displayed speech recognitionresult.

Turning now to FIG. 3, there is illustrated a speech synthesis system 66according to an embodiment of the present invention. In this illustratedexample, a digitized representation of a speech sound wave is obtainedat the capture and conversion unit 68, the output of which is deliveredto an inversion calculation module 70 and a feature extraction module72. A database 74 of stored voicing features is arranged to receive theoutput from the feature extraction module and, as will be describedfurther below, produce an output serving to influence control a voicesynthesizer module.

As with the speech recognition system illustrated in FIG. 2, the outputfrom the inversion calculation module 70 of the speech synthesis system66 produces a general potential function 76, which is delivered to bothan optimizer module 78 and the aforementioned speech synthesizer module80. A stream of the binary parameters is output from a database 84 ofvocal-tract renormalized, speaker-independent, generally binary strings,which output is influenced by the output from the database 74 of voicingfeatures and which, in combination with the general potential function76, serves to control the speech synthesis at the speech synthesizermodule 80 so as produce a synthesized voice output 82.

With regard to FIG. 4, there is illustrated a voice identificationsystem 86 which, as with the embodiment of the present inventionillustrated in FIG. 3, employs a capture and conversion module 88arranged to deliver a signal to each of an inversion calculation module90 and a feature extraction module 92. Again, the inversion calculationmodule serves to generate a general potential function 94 which, incombination with the voicing feature 96 output from the featureextraction module 92 is delivered to a comparator module 98 which isalso arranged to receive an output from a database 97 of voice samplesof known individuals.

The comparison of the speaker-dependent part of the potential function94 with the voice samples in the database 97 relating to knownindividuals, serves to provide for a voice identification output result100.

Turning now to FIG. 5, there is illustrated an example of a speechcompression system according to an embodiment of the present invention.Here, the output from a capture conversion module 104 is again deliveredto an inversion calculation module 106 so as to derive a potentialfunction and the output from the inversion calculation module 106 isdelivered to an s optimizer module 108. The optimizer module 108 outputis delivered to a database for comparison with vocal-tract renormalized,speaker-independent, generally binary strings so as to produce a binaryparameter stream representative of the incoming speech signal.

Such a binary representation of the incoming speech signal can thenadvantageously exhibit a compressed format so as to provide for therequired speech compression.

The processing relating to generation of the potential function relativeto the inversion calculation, and the generating of the potentialfunction, is now described in further detail.

It has previously been noted that the pressure P(x), and area, S(x),functions, appearing in the Webster equation, must together obey theprinciple of conservation of energy such that, averaged over a period,τ,<P′ ²(x, t)>S(x)=const.   (1)

Defining a new variable, the wavefunction, ψ,ψ(x,t)=P′(x,t)S(x)^(1/2)   (2)

thus removes much of the predictable fluctuation of pressure with axialdistance and elucidates the physically significant dispersive phenomena.Substitutions for P′(x,t) within the Webster equation then result in theKlein-Gordon form: $\begin{matrix}{\frac{{\partial^{2}\Psi}\quad\left( {x,t} \right)}{\partial t^{2}} = {c^{2}{\left\{ {\frac{{\partial^{2}\Psi}\quad\left( {x,t} \right)}{\partial x^{2}} - {U\quad(x)\quad\Psi\quad\left( {x,t} \right)}} \right\}.}}} & (3)\end{matrix}$

Equation (3) has the form of a wave equation holding under theassumptions of one-dimensional propagation in a compressible fluid, inthe non-viscous approximation, where ψ²(x,t) is directly propagation tothe potential energy per unit length of fluid. The potential function,U(x), is defined in terms of a continuously defined area function S(x),that is, $\begin{matrix}{{U\quad(x)} = {\frac{d^{2}S\quad{(x)^{1/2}/{dx}^{2}}}{S\quad(x)^{1/2}}.}} & (4)\end{matrix}$

Two cases of special interest arise, namely those of the positive(“barrier”) and negative (“well”) potentials.

For a piecewise-continuous potential function, U₀, where U₀>0,time-independent solutions, ψ(x), are found in terms of a dispersivewave number, k′, such that k′=(k²−U₀)^(1/2). A wave propagates withincreased phase velocity over such a barrier, and is exponentiallydecaying within it, that is, for k²<U₀.

Given U₀, an underlying area function can be recovered from equation (4)only for two known initial conditions on S(x)^(1/2). For a known area,S(0), at the glottal boundary and zero initial gradientdS(x)^(1/2)/dx=0, a particular solution is found such thatS(x))^(1/2) =S(0)^(1/2)cosh U ₀ ^(1/2) x,  (5)

describing a section of catenoidal horn.

For U₀<0, the dispersion is then such that k′=(k²+|U₀|)^(1/2). A wavepropagates, with decreased phase velocity over such a barrier, and maybe bound within it. For the initial conditions as in the situation U₀>0above, it is found thatS(x)^(1/2) =S(0)^(1/2)cos|U₀|^(1/2) x.   (6)

In general, however, any particular potential function will map to aninfinite “equivalence set” of area functions. This is illustrated inFIG. 6 for a single barrier of 1 mm width and height 10⁵m⁻², terminatinga tract of length 175 mm. FIG. 7 shows the effect on the area functionof preceding such a terminating barrier with a well of the samedimensions, at varying separation of the pair. Localized constrictions,of degrees increasing with separation length, are obtained. A variety ofacoustical effects, not evident in standard accounts, accompany thetransition to an approximately single resonator configuration. Followingthe analysis, simple mathematical constraints were predicted for theheight and width of acoustical barriers and wells within a vocal-tract.Constraints were then sought on the positioning of such potentials. Thiswas achieved through a first-order, time-independent perturbationanalysis.

In contrast to the standard perturbative account, the following analysistakes account of small dispersions, rather than changes in tract area.Consider a small perturbation around resonances sk_(n), such thatδk_(n)=k′_(n)−k_(n), for k′_(n)=(k_(n) ² −U ₀)^(1/2). For a tract oflength l, the corrected eigenfunctions, ψ_(cn)(x), may be written$\begin{matrix}{{\Psi_{cn}(x)} = {A_{n}\cos\quad{\left\{ {\left\lbrack {\frac{\left( {{2n} + 1} \right)\quad\pi}{2l} + {\delta\quad k_{n}}} \right\rbrack\quad x} \right\}.}}} & (7)\end{matrix}$

The corrected potential energy per unit length e_(cpn), can be writtento first order as $\begin{matrix}{{{e_{cpn}(x)} = {\frac{A_{n}^{2}}{4\rho_{0}c^{2}} \times \left( {1 + {\cos\quad\begin{matrix}{\left\lbrack \frac{\left( {{2n} + 1} \right)\quad\pi\quad x}{l} \right\rbrack -} \\{2\delta\quad k_{n}x\quad{\sin\quad\left\lbrack \frac{\left( {{2n} + 1} \right)\quad\pi\quad x}{l} \right\rbrack}}\end{matrix}}} \right)}};} & (8)\end{matrix}$

Thus defining a first-order perturbation, δ_(epn)(x), to the potentialenergy. $\begin{matrix}{{\delta\quad{e_{pn}(x)}} = {{- \frac{A_{n}^{2}}{2\rho_{0}c^{2}}}\delta\quad k_{n}x\quad{{\sin\quad\left\lbrack \frac{\left( {{2n} + 1} \right)\quad\pi\quad x}{l} \right\rbrack}.}}} & (9)\end{matrix}$

Since δk_(n) is positive for a well but negative for propagation above abarrier, it can be shown that (a) the perturbative term may be in or outof phase with the radiation pressure, thus strengthening or weakeningthe resonances, respectively; and that (b), a perturbing well or barriermay, by Ehrenfest's theorem, raise, lower or have no effect on aneigenfrequency, depending on the interaction with the phase of thesinusoidal term. These results can be demonstrated by assuming aperturbation δk_(n)=±1m⁻¹ which entails U₀˜±(20 m⁻²) at the firsteigenfunction of a tract of length 175 mm.

It is found from equation (7), and illustrated in FIG. 8, thatconstructive effects on the first eigenfunction occur for a barrierperturbation for 0<x<1/2, and a well for 1/2<x<1, since theperturbations are then in phase with the radiation pressure. FIG. 9illustrates a destructive flattening effect when the reversepotential-function configuration is adopted.

Referring now to FIG. 10, there are shown examples ofpiecewise-continuous (bitwise) potential-function strings, where thenotation refers to predicted mathematical constraints on barrier andwell potentials and SWP positions. The results for a 6-bit vocal-tractmodel are shown of which 4 bits are orthogonal and two exhibitstatistical dependencies, and also examples from 25 a vowel system.

The six-bit table of FIG. 10 illustrates how it is possible todifferentiate between all linguistic classes for a full 25-vowel systemfor example round vowels at the 6^(th)-bit, front vowels at the 3^(rd)and 5^(th) bits, low vowels at the first and second bits and rhoticvowels at the 4^(th) bit. FIG. 11 is a vowel corresponding to FIG. 10.

It should therefore be reiterated that, depending on the initialconditions at the glottis, many area functions correspond to any givenpotential-function string. That is, there is a many-to-one mappingbetween the area and potential functions. Nevertheless, general commentscan be made about possible gestural correlates of the bitwise strings.For example, the 1st and 2nd bits, identified with the non-high vowels,denote a positive tract curvature (most simply, an expansion) at the1/10 and 21/5—approximately glottal and pharyngeal—regions. The presenceof these bits suggests, for example, a retraction of the tongue root.The 3rd and 5th bits correspond to potential-function wells spanning thefront half of the vocal tract, and are in line with a constrictionextending over the hard palate, typical of the front vowels. The 4th bittypifies a shorter constriction centered in the same region, indicativeof the central vowels.

As an alternative to the five or six binary parameters previouslydiscussed for handling the consonant sounds the possibilities forderiving appropriate parameter representation of such sounds from the6-bit table illustrated in FIG. 10 is also recognized.

This possibility arises through the identification of three furtherclass of sounds from the aforementioned 6-bit table and which comprisenasalised vowels, laterals such as “l-type” sounds, and also rhotic“r-type” sounds referred to generally as steady-state sonorants and asdiscussed further as follows.

With reference to FIG. 10, it should be appreciated that nasalisedvowels can be obtained from a 6-bit string 01xxxx, wherein x refers toeither a 0 or a 1. That is to say, any entry illustrated in FIG. 10 andbeginning 10xxxx can include a counterpart 01xxxx which indicates thenasalised version of that entry. Thus, ten nasalised vowels can beobtained from the table illustrated in FIG. 10. The notation employedserves to imply a barrier of approximate width and height 1 mm and10⁴m⁻² respectively at the 21/5 position, and also an implied well atx=l at the limit of bound state solutions for which |U_(o)|Δ²=π²/4.

With regard to the above-mentioned laterals, i.e. the “l-type” sounds,these will be obtained from the 6-bit string xx011x and my be consideredas “clear” or “dark” depending on other bits in the string.

Likewise, the rhotic, or “r-type” sounds can be obtained from the 6-bitstring xx0011 and can also be considered either “clear” or “dark”depending upon other bits in the string.

Yet further, it is appreciated that it is now possible to state anotherbinary element indicating the absence of periodic voicing at theglottis, which of course would be the default case, and also thepresence of aperiodic energy, which characterizes the voiceless soundsand those arising with a so-called breathy voice. The same binaryelement could also serve to code the distinctive fundamental frequencyin voiced sounds such as high tones in tone languages such as Chinese.In such a case, a baseline tone is taken as the default position andconsidered to correspond to the voicing of sonorants, in nontonelanguages. Thus, referring again to the table of FIG. 10, it will beappreciated that any string can be proceeded by another entry x(xxxxxx)which may be 0 for a default case for the voicing of vowels/sonorantsand with a baseline tone in tone language, or which may be a 1 forvoiceless or breathy sonorants, or high tone in tone language.

This additional binary parameter can be considered as a voicingparameter since no particular reference is made to the potentialfunction.

Thus, as will be appreciated from the above, it is considered that allsonorants, i.e. vowels, laterals and rhotics whether nasalised, voiced,voiceless, or breathy voiced sounds can be represented by way of sevenbinary parameters and it is likewise thought that all remainingconsonants can be represented by means of only another two or threebinary parameters so that in the order of nine parameters are thenrequired to provide for full representation.

As compared with the previous discussion concerning the use ofadditional five-six parameters for consonant sounds, it will beappreciated that reliance on the table of FIG. 6 in order to provide theabove-mentioned further three classes of sounds leads to yet furtherimprovements in accuracy and efficiency.

Compared to traditional area function tract description therefore, thepotential-function formalism has the unique advantage of quantifying thephysics of speech production on a level that is both more abstract andcompact. Most importantly, the bitwise strings predicted by mathematicalanalysis have been found to have clearly phonological properties, whilstmapping deterministically to the phonetic level, both aural and in termsof a tract area function. The proposed six-bit model for vowel sounds,together with the five/six bit consonantal parameters allow for asophisticated implementation of an intermediate representation,specifically a phonetics phonology interface, in an automatic speechrecognition architecture such as that discussed herein.

As noted above with regard to speech production it is appreciated withinthe present invention that just six binary parameters, stated in termsof a potential-function string, are sufficient to synthesize theacoustic characteristics of a full 25-vowel system and as described bythe standard phonetic alphabet. The addition of a small number of extrabinary parameters, generally in the order of five or six, allows thedescription of the consonants and other tokens of the speech stream.

A more recently developed inversion technique shows that a uniqueinverse mapping exists between the speech signal and the vocal tractpotential function. The general speech-recognition problem then can bereduced to finding a best fit between the recovered potential functionand other non-vowel parameters and “template” binary strings, whileother speech processing applications also will be based upon the use ofthe potential function as a model for speech generation.

Considering now in more detail the inversion calculation, and returningto equation (4) above, it should be noted that the document Benade, A.H. and Jansson, E. V; On Plane and Spherical Horns and Non-UniformFlare: I Theory of Radiation, Resonance Frequencies, and ModeConversion; Acustica; Vol. 31 (1974), suggests that the function U(x)plays a similar role to the potential energy function of theSchroedinger equation of quantum mechanics and that it provides completeinformation about the frequency-dependent reflection (R(k)) andtransmission (T(k))coefficients of the acoustic waves in the tract wherethe wave-vector k is equal to c divided by the frequency w. However, theKlein-Gordon equation has not been used before in the context of speechacoustics and differs from the Schroedinger equation in that the timederivative appears in second rather than in first order. This makes acrucial difference to the time-dependent behavior of the speech waves.The potential function, however plays a similar role as a scatteringsource in both of these equations when waves of single Fourierfrequencies are considered.

For a rectangular barrier, the transmission and reflection coefficientsare obtained by the method of matching the wave function T(x,t), and itsfirst derivatives at the barrier edges. The transmission characteristicsof such barriers are therefore obtained very directly.

By modeling the tract as a series of barriers of this simple shape it ispossible to solve the Klein-Gordon equation analytically and thus obtainthe Green's function G_(f)(1|0|w) for the response of a tract of lengthL, taken at an arbitrary distance, l, outside it, to a volume-velocityinput C_(w)e^(iwt) at the glottis. This is equal to the pressure, whichwould be measured by a microphone placed at this position. In terms ofthe algebraically calculated reflection and transmission coefficients itis found that $\begin{matrix}{{G_{f}\left( {l{0}w} \right)} = {C_{w}\frac{T\quad(k)}{1 - {R\quad(k)}}{\mathbb{e}}^{{- {\mathbb{i}}}\quad{kl}}}} & (10)\end{matrix}$

where C_(w) is the Fourier coefficient of the glottis model, for which,for example, we can use any one of a number in the literature such asthe one by Klatt.

The following represents a proof due to Aktosun [Aktosun, T.Construction of the half line potential from the Jost function. IMAPreprint No. 1926 (2003)] that the required inverse mapping can beachieved and further represents one example of how the inversion can beachieved in a frequency dependent manner.

It should of course be appreciated that the invention is not restrictedto such details and that other methods can be used.

To invert the measured microphone signal to obtain the potentialfunction we assume, on the basis of our numerical research, that thepotential U does not support any bound states. It is real valued,vanishes for x<0, includes no delta distributions, and belongs to L¦(R).R denotes the ponts of the real line and by L¦ (R) we denote theLebesque-measurable potentials U such that ƒ^(∞) _(−∞)dx(1+|x|)|U(x)| isfinite. Under these conditions the following solution has been derived.

The scattering states at frequency w of the Klein-Gordon equationcorrespond to its solutions behaving like e^(ikx) or e^(−ikx) as x→±∞,and such states occur for kεR\{0}, that is in R excluding the zeropoint. Among these is the Jost solution from the left, f₁(k,x),satisfying the boundary conditions.f ₁(k, x)=e ^(ikx)[1+o(1)], f′ ₁(k, x)=(k, x)=ike ^(ikx)[1+o(1)], x→+∞.  (11)

The transmission coefficient, T, and the reflection coefficient from theleft, R, are related to the asymptotics of f₁(k,x) as $\begin{matrix}{{{f_{l}\left( {k,x} \right)} = {{\frac{1}{T\quad(k)}{\mathbb{e}}^{{\mathbb{i}}\quad{kx}}} + {\frac{R\quad(k)}{T\quad(k)}{\mathbb{e}}^{{- {\mathbb{i}}}\quad{kx}}} + {o\quad(1)}}},\left. x\rightarrow{- \infty} \right.,} & (12) \\{{{f_{l}^{\prime}\left( {k,x} \right)} = {{\frac{ik}{T\quad(k)}{\mathbb{e}}^{{\mathbb{i}}\quad{kx}}} - {\frac{{ik}\quad R\quad(k)}{T\quad(k)}{\mathbb{e}}^{{- {\mathbb{i}}}\quad{kx}}} + {o\quad(1)}}},\left. x\rightarrow{- {\infty.}} \right.} & (13)\end{matrix}$Since it can be assumed that U(x)=0 for x<0, it then follows that$\begin{matrix}{{{f_{l}\left( {k,x} \right)} = {{\frac{1}{T\quad(k)}{\mathbb{e}}^{{\mathbb{i}}\quad{kx}}} + {\frac{R\quad(k)}{T\quad(k)}{\mathbb{e}}^{{- {\mathbb{i}}}\quad{kx}}}}},{x \leq 0},} & (14) \\{{{f_{l}^{\prime}\left( {k,x} \right)} = {{\frac{ik}{T\quad(k)}{\mathbb{e}}^{{\mathbb{i}}\quad{kx}}} - {\frac{{ik}\quad R\quad(k)}{T\quad(k)}{\mathbb{e}}^{{- {\mathbb{i}}}\quad{kx}}}}},{x \leq 0.}} & (15)\end{matrix}$A determination of U from [1−R(k)]/T(k) is then obtained as follows.From equation 15 we see that a determination of [1−R(k)]/T(k) isequivalent to a determination of f (k,0). It should be noted that theamplitude of the reciprocal of this quantity is related to the real partof [1+R(k)]/[1−R(k)]. From this $\begin{matrix}\begin{matrix}{{{Re}\quad\left\{ \frac{1 + {R\quad(k)}}{1 - {R\quad(k)}} \right\}} = \frac{1 - {{R\quad(k)}}^{2}}{{{1 - {R\quad(k)}}}^{2}}} \\{= \frac{{{T\quad(k)}}^{2}}{{{1 - {R\quad(k)}}}^{2}}} \\{{= \frac{k^{2}}{{{f_{l}^{\prime}\left( {k,0} \right)}}^{2}}},{k \in R},}\end{matrix} & (16)\end{matrix}$wherein the fact that |T(k)|^(2+|R(k)|) ²=1 for kεR has been employed.It should be appreciated that [1+R]/[1−R] is analytic in the upper halfof the complex plane, C⁺, continuous in its closure C⁺, and 1+₀(1/k) ask→∞ in C⁺. Thus, by the Schwarz integral formula (the poisson integralformula for half line) L. Ahlfors, Complex analysis, 2^(nd) ed.,McGraw-Hill, New York, 21966, the quantity [1+R)/[1−R] is uniquelydetermined by its real part. Thus, R for kεR is uniquely determined byknowledge of |f (k,0)| for kε[0,+∞). Equivalently, U is thereforeuniquely determined by [1−R]/T. Letting $\begin{matrix}{{{ɛ\quad(k)\text{:}} = {\frac{1 + {R\quad(k)}}{1 - {R\quad(k)}} - 1}},} & (17)\end{matrix}$then from equation 16 if can be determined that $\begin{matrix}{{{{Re}\left\{ {ɛ(k)} \right\}} = {\frac{k^{2}}{{{f_{l}^{\prime}\left( {k,0} \right)}}^{2}} - 1}},{k \in R},} & (18)\end{matrix}$and hence, by the Schwarz integral formula, $\begin{matrix}{{{ɛ(k)} = {\frac{i}{\pi}{\int_{- \infty}^{\infty}\quad{\frac{\mathbb{d}t}{k + {i\quad 0^{+}} - t}\left\lbrack {\frac{t^{2}}{{{f_{l}^{\prime}\left( {t,0} \right)}}^{2}} - 1} \right\rbrack}}}},{k \in {\overset{\_}{C^{+}}.}}} & (19)\end{matrix}$Once ε(k) constructed, R(k) is obtained as $\begin{matrix}{{{R(k)} = \frac{ɛ(k)}{2 + {ɛ(k)}}},{k \in {\overset{\_}{C^{+}}.}}} & (20)\end{matrix}$

Having determination R(k) for kεR, U can be constructed by any one ofthe available methods K. Chadan and P. C. Sabatier, Inverse Problems inQuantum Scattering Theory, 2^(nd) ed., Springer, N.Y., 1989, T. Aktosunand M. Klaus, Inverse theory: problem on the line, Chapter 2.2.4 in:Scattering, eds. E. R. Pike and P. C. Sabatier (Academic Press, London,2002).

For the above it will be appreciated that, as one example, a speakeridentification process can comprise means to capture an incoming voicesignal, for example from a microphone or telephone line; means toprocess the signal electronically to generate a time varying series ofbinary vocal-tract potentials and associated non-vowel binaryparameters; means to refine the signal to revoke the speaker-independentspeech components; and means to compare the residual signal with adatabase of such residual features of known individuals. Also, means tocompare the aforementioned binary strings with a table known parsablespeech tokens can be provided along with means to parse the token streamto confirm and/or refine interpretation using other grammar and/orcontext rules; and means to output the interpretation, for example, acomputer screen or printing device.

This further example involves again the speaker-independent (bitwise)part of the recovered potential function and could be employed forexample, in telephony, particularly mobile telecoms. An importantcontext is the military field, where the need to transmit both speechand e-text leads to shared-bandwidth problems. Recent estimates are that72 bps speech compression is required, in comparison to current 2.4kbps. The potential function system will operate at lower than 90 bps.

The removal of the speaker-independent part of the speech in this wayfacilitates the analysis of the rest of the signal forspeaker-identification purposes.

Speech processing security applications are commonly employed insituations involving telephony where communication lines can beautomatically interrogated. A method according to the present inventionhas the benefit of remote operation. This comprises most favourablywith, for example, fingerprint and iris patterning which require closecontact for the compilation of the initial database of samples andrelatively close contact at the automatic scanning stage.

The speech-recognition process uses the speaker-independent part of therecovered binary parameters; primarily the binary strings noted abovefor the vowels but also the additional non-vowel parameters. Asimplemented in a grammatical parser in place of, for example, acurrently used cepstral coefficients, the applications are extremelywide, as has already been indicated.

Finally, with regard to speech synthesis, an example of the inventioncomprise means to generate a binary speech token stream for thespeaker-independent component of the message to be synthesized, forexample, from a database of words or phrases; means to convert thebinary steam to a band-limited analogue electrical signal; and means toconvert this signal to audible speech such as a loudspeaker

Applications can relate to text-to-speech systems, for personalcomputing applications and information dissemination including, forexample, the speaking clock or railway tannoy systems.

1-51. (canceled)
 52. Speech processing apparatus arranged for the inputor output of a speech data signal and including a function generatingmeans arranged for producing a representation of a vocal-tract six bitpotential function for vowel identification representative of a speechsource.
 53. An apparatus as claimed in claim 52, and including means forderiving single-bit consonantal features.
 54. An apparatus as claimed inclaim 53, wherein consonantal sounds are defined as five or sixadditional bits.
 55. As apparatus as claimed in claim 52, and arrangedfor deriving linguistic parameters representing sonorant sounds from thesaid 6-bits.
 56. An apparatus as claimed in claim 55, wherein thesonorant sounds comprise one or more of nasalised vowels, laterals andrhotics.
 57. An apparatus as claimed in claim 55, and arranged toinclude a further binary parameter with the said 6-bits serving toindicate the absence of periodic voicing at the glottis and/or thepresence of aperiodic energy.
 58. An apparatus as claimed in claim 55,wherein linguistic parameters in the order of two or three additionalbits are defined for consonant sounds.
 59. An apparatus as claimed inclaim 52, and including means for specifying the said potential functionas a general function having parameters serving to discriminate betweenphonemes.
 60. An apparatus as claimed in claim 52, wherein the saidfunction generating means is arranged to perform an inversion algorithmderived from a Green's function solution for a vocal-tract wavefunction.
 61. An apparatus as claimed in claim 52, wherein the saidfunction generating means is arranged to produce potential functionstrings.
 62. An apparatus as claimed in claim 52, and including meansfor discriminating between speaker dependent and speaker independentparts of the potential function.
 63. Speech recognition apparatusincluding means for receiving a speech data signal and speech processingapparatus as claimed in claim 52, and further including means forconducting a template matching procedure on the output of the functiongenerating means.
 64. An apparatus as claimed in claim 63, and includingmeans for performing an inversion calculation on the said speech datasignal so as to derive the potential function.
 65. An apparatus asclaimed in claim 63, wherein the said template matching procedure isarranged to be conducted on a speaker independent part of the potentialfunction.
 66. An apparatus as claimed in claim 63, wherein the saidmeans for conducting the template matching procedure is arranged toprovide comparison to binary potential function strings stored inlook-up tables, and which serves to achieve vocal-tract lengthnormalization.
 67. An apparatus as claimed in claim 66, and includingparsing means arranged to receive phoneme identifiers output from thetemplate matching means.
 68. Voice identification apparatus includingmeans for receiving a data signal, and speech processing apparatus asclaimed in claim
 52. 69. An apparatus as claimed in claim 68, andincluding means for performing an inversion calculation on the saidspeech data signal so as to derive the potential function.
 70. Anapparatus as claimed in claim 69, and including means for performing amatching operation on stored data identifying individual and on thebasis of speaker-dependent parts of the potential function.
 71. Speechsynthesis apparatus including speech processing apparatus of claim 52,and including means for receiving speech parameters and forreconstructing a speech sound wave on the basis of the said potentialfunction which serves to produce a speech token stream.
 72. An apparatusas claimed in claim 71, and arranged such that the speech sound wave isreconstructed having regard to speaker-independent parts of thepotential function.
 73. An apparatus as claimed in claim 71, andincluding means for converting a stream of speech tokens into ananalogue speech signal.
 74. Speech signal compression apparatusincluding means for receiving a speech data signal, and speechprocessing apparatus as claimed in claim
 52. 75. An apparatus as claimedin claim 74, and including means for performing an inversion calculationon the speech data signals so as to derive the potential function. 76.An apparatus as claimed in claim 74, and including template matchingmeans for receiving the output from the function generating means andfor reconstructing speaker independent parts of the potential functionas compressed speech data.
 77. An apparatus as claimed in 52, whereinthe said function generating means is arranged to generate a timevarying series of binary vocal-tract potentials and associated non-vowelbinary parameters.
 78. A speech processing method for processing inputor output speech data and including the step of generating arepresentation of a vocal-tract six bit potential function for vowelidentification representative of a speech source.
 79. A method asclaimed in claim 78, and including the step of specifying the saidpotential function as a general function having parameters serving todiscriminate between phonemes.
 80. A method as claimed in claim 78, andincluding the step of deriving single-bit consonantal features.
 81. Amethod as claimed in claim 78, and including the definition ofconsonantal sounds as five or six additional bits.
 82. A method asclaimed in claim 78, and including the step of deriving linguisticparameters representing sonorant sounds for the said 6-bits.
 83. Amethod as claimed in claim 80, wherein the sonorant sounds comprise oneore more of nasalised vowels, laterals and rhotics.
 84. A method asclaimed in claim 80, and including the step of including a furtherbinary parameter with the said 6-bits serving to indicate the absence ofperiodic voicing at the glottis and/or the presence of aperiodic energy.85. A method as claimed in claim 82, wherein linguistic parameters inthe order of two or three additional bits are defined for consonantsounds.
 86. A method as claimed in claim 78, and including the step ofperforming an inversion algorithm derived from a Green's functionsolution for the vocal-tract wave function.
 87. A method as claimed inclaim 78, and including the step of producing the vocal-tract potentialfunction as potential function strings.
 88. A method as claimed in claim78, and including the step of discriminating between speaker-dependent,and speaker-independent, parts of the potential function.
 89. A speechrecognition method including the step of receiving a speech data signaland further including the processing steps of claim 78 and also the stepof conducting a template matching procedure on the vocal-tract potentialfunction.
 90. A method as claimed in claim 89, and including the step ofperforming an inversion calculation on the speech data signal so as toderive the potential function.
 91. A method as claimed in claim 89,wherein the template matching procedure is conducted on aspeaker-independent part of the potential function.
 92. A method asclaimed in claim 89, wherein the step of conducting the templatematching procedure includes the step of providing a comparison withbinary potential function strings stored in look-up tables.
 93. A methodas claimed in claim 92, and including the step of parsing receivedphoneme identifiers resulting from the template-matching step.
 94. Avoice identification method including the step of receiving a speechdata signal and including speech-processing steps such as defined inclaim
 78. 95. A method as claimed in claim 94, and including the step ofspecifying the said potential function as a general function havingparameters serving to discriminate between phonemes.
 96. A method asclaimed in claim 95, and including the step of performing a matchingoperation on the stored data identifying individuals, and on the basisof speaker-dependent parts of the potential function.
 97. A speechsynthesis method including the processing steps of claim 78, and furtherincluding the step of receiving speech parameters and for reconstructinga speech sound wave on the basis of the said potential function.
 98. Amethod as claimed in claim 97, and including the step of reconstructingthe speech sound wave having regard to speaker-independent parts of thepotential function.
 99. A method as claimed in claim 97, and includingthe step of converting a stream of speech tokens into an analogue speechsignal.
 100. A speech signal compression method, including the steps ofreceiving a speech data signal and further including the speechprocessing steps of claim
 78. 101. A method as claimed in claim 100, andincluding the step of performing an inversion calculation on the speechdata signals so as to derive the potential function.
 102. A method asclaimed in claim 100, and including the step of receiving the result ofthe potential function and for delivering the same to template matchingmeans and for reconstructing speaker-independent parts of the potentialfunction as compressed speech data.